Babies who wake up during the night to be fed are trying to delay the birth of a younger brother or sister, researchers claim.

A Harvard University study said that shorter delays between the birth of siblings is linked to higher mortality rates, particularly in areas with widespread infectious diseases and scarce resources.

This means that a Darwinian tactic has developed in breast feeding children, in an attempt to reduce competition to improve their survival, it is claimed.

During the first six months after birth, breast feeding acts as a natural contraceptive.

The study was carried out by Harvard University

Professor David Haigh, who led the study, said babies are most likely to wake often in the night at around six months, which is around the time when mothers are becoming fertile again.

In a paper published on Thursday in the journal Evolution, Medicine and Public Health, he said: "Natural selection will have preserved suckling and sleeping behaviours of infants that suppress ovarian function in mothers, because infants have benefited from delay of the next birth.

"Maternal fatigue can be seen as an integral part of an infant’s strategy to extend the inter-birth interval."

He added: "More frequent and more intense nursing, especially at night, is associated with prolonged infertility."

Evidence from babies with Angelman syndrome suggests it may be fathers' genes that are responsible, he said.